Natural gas liquids are hydrocarbons containing two or more carbon atoms which are normally found in natural gas reservoirs. Examples of natural gas liquids are ethane, propane and butane. When recovering natural gas, i.e. methane, from a natural gas reservoir, it is desirable to separate the natural gas liquids from the natural gas and recover the two separately. This is because natural gas liquids have a higher economic value than methane for use as fuel such as propane or liquified petroleum gas, or for use as chemical feedstocks. When nitrogen is also present in the natural gas reservoir, it is desirable to separate the nitrogen from the hydrocarbons while not adversely affecting the separation of natural gas liquids from the natural gas. A reservoir may have a naturally occurring nitrogen content of from 0 to 90 percent, generally from 3 to 5 percent.
As hydrocarbon resources become scarcer and more difficult to recover, secondary recovery operations are becoming more widespread. Such secondary recovery operations are commonly referred to as enhanced oil recovery (EOR) and enhanced gas recovery (EGR) operations. One such secondary recovery technique involves the injection of a gas which does not support combustion into a reservoir to raise reservoir pressure in order to remove hydrocarbons which cannot be removed from the reservoir by natural reservoir pressure. A commonly used gas for this process is nitrogen because it is relatively abundant and inexpensive and can be produced in large quantities at the reservoir site.
The injection of nitrogen into the reservoir will result, over time, in the presence of increased concentrations of nitrogen in the natural gas recovered from the reservoir. The nitrogen concentration of the fluid recovered from the reservoir can be from the naturally occurring concentration to as high as 90 percent or more. Furthermore the nitrogen concentration of the recovered gas does not remain constant, but tends to increase over time as more and more nitrogen is employed to keep reservoir pressure at a point where recovery can proceed. This has an adverse effect on the recovery of natural gas liquids separate from the natural gas.
The increasing concentration of nitrogen in the wellhead stream complicates the effective separation of natural gas liquids from natural gas because a process which may be effective at a relatively low nitrogen concentration, such as around 5 percent may be ineffective at a high nitrogen concentration, such as greater than 50 percent. Thus a process to separate natural gas liquids from nitrogen containing natural gas recovered from a reservoir which has undergone nitrogen injection must have sufficient flexibility to effectively carry out the separation over a wide range of nitrogen concentrations.
It is therefore an object of this invention to provide an improved process for separating natural gas liquids from natural gas which also contains nitrogen.
It is another object of this invention to provide a process to effectively separate natural gas liquids from nitrogen-containing natural gas having a relatively high nitrogen concentration.
It is a further object of this invention to provide a process to effectively separate natural gas liquids from nitrogen-containing natural gas wherein the nitrogen concentration may vary from the naturally occurring concentration to as much as 90 percent or more.